It opens with "Who are these two guys and why are they fighting?" They stop for some explaining. Cut to the castle for more explaining about the usual shogun, lords, someone is dying, the succession, and it all comes down to "Get the letter!" Four ronin are hired to protect it from the ninjas. The samurai are displeased about that. The story continues with a few misdirections and twists as the good guys overcome the ninja threat.
The ninja and samurai movies from the 1960s Japan all start to look alike after watching about ten or twelve of them. I rate this one as above average for the year and genre. What makes this one not run of the mill and forgettable? Mostly because it avoids the typical faults. The story is straight forward and does not deviate. There are plenty of characters but not too many and the characters are distinct. (Sometimes it is an effort to get it straight exactly who is who.) Most important is the pacing. The scenes get straight to the point and there are many extended cuts. (These are long scenes, action or dialog that can run two or three minutes straight before the camera cuts away.) These extended cuts keep up the pace of the movie.
For fans of the genre I rate it just above average. If you are new to this sort of movie it might be better to start with the classics first such as The Seven Samurai or the Lone Wolf and Cub series or the Zatoichi series.